Mixing breeds and different age chicks?

I am new to the world of chicken parenting and would just like to get some opinions on mixing breeds, is it okay?
Also, is it okay to mix chicks of different ages?

The people at the feed store we got them from said that mixing breeds was fine as long as they're all close to the same age/size.
So, we chose a varity of chicks. Our littlest baby passed away early yesterday morning, reason unknown. All of the others seem to be fine and healthy. My husband and I are pretty bummed over the passing (it was our favorite baby). We were thinking about getting 1 or 2 more but don't know, is adding more breeds to the flock a bad idea? And if we got a couple more babies, are the others too old now to accept it and treat it nicely?

So far no problem with my chicks. 5-3 week old jersey giants with my 6 week old RIR & Barred Rocks. They eat & drink together & are cuddled up sleeping. I have them indoors in an old ferret cage with hay & a light, sleeping now.

I have 4 Rhode Island Reds and 4 Buff Orpingtons who are all a day old, however I added 1 ameraucana to the flock of baby chicks today and I'm a little nervouse on whether or not all of the others will eventually pick on the one when they het older. Has anyone had problems with this??

I added two day old cochin chicks to three three day old chicks who are americana, buff orpington and Plymouth rock. Everyone was fine and after 3 weeks they all get along very well. I hope this helps!